Yes, unfortunately you can only purchase Super Priority at the time you submit and pay for the application. It is not possible to add it later.
There are only a limited number of Super Priority available each day (about 50 I think), and so if all the slots are gone at the time you submit, you will not be given the option to purchase it.
So, the way to get it is to log on to submit the application at 1am when the new slots are released and hope for the best. If there are none available, you can just keep trying each night.
In your case though, you have already submitted and paid for your application which will give you Standard processing only.
So, your options are:
- keep the standard processing, which can take up to 6 months
Or
- withdraw your application before attending biometrics and apply for a refund. Then start the application over again and resubmit and pay for it, trying to get Super Priority
The withdrawing and starting over can be done right away, but the refund can take a few weeks to come through after that.
See here for how to withdraw the application:
https://www.gov.uk/cancel-visaSent from my iPhone using Tapatalk